Motors are employed as a driving source of various apparatuses such as video and audio apparatuses, office automation apparatuses, home appliances, transporting devices, factory automation devices. These apparatuses and devices have been equipped with more functions year by year, thus a number of motors employed in these apparatuses and devices has increased accordingly. At the same time, the apparatuses and devices have been downsized and done their jobs at a higher speed. The market therefore has requested that motors output a greater power with a smaller body.
Various types of motors are used, such as an induction motor, a dc motor with a brush, a stepping motor, and a brush-less motor. The present invention relates to the brush-less motor.
FIG. 31A and FIG. 31B show a first example, i.e., a conventional brush-less motor. In FIG. 31A, a stator comprises core 103 and coils 104 wound on salient poles of core 103. A rotor comprises magnets 101 and shaft 112 to which magnets 101 are mounted. Core 103 faces magnets 101 with a predetermined space inbetween. Both the ends of shaft 112 are journalled by bearings 113.
In FIG. 31B, magnets 101 are magnetized to form eight poles. Core 103 facing magnets 101 has six salient poles uniformly spaced and wound with coils 104. Adjacent salient poles have a phase difference of 120 degrees in electrical angles. Salient poles 105-1 and 105-4, both being in the same phase, are assigned to phase U, salient poles 105-2, 105-5 also in the same phase are assigned to phase V, and salient poles 105-3, 105-6 in phase are assigned to phase W Coils 104 wound on each salient pole, namely phase U coil, phase V coil and phase W coil, are powered and controlled detecting a rotor position, so that the rotor is driven.
As shown in this first example of the conventional brush-less motor, edges of respective salient poles are flat in general.
On the other hand, a motor in which an edge of each one of salient poles is toothed, i.e., plural small teeth are provided at the edge of a pole (vernier structure), is available as a hybrid (HB type) stepping motor, and this HB type motor is commercialized and provided to general uses.
A stepping motor of HB type generally used is described hereinafter. FIG. 32A and FIG. 32B show a structure of a second example, i.e., a conventional HB type stepping motor. In both the Figs., magnet 201 is magnetized two poles (N and S poles) along a rotary shaft. Rotor iron-core 211 is magnetized N pole or S pole at an entire upper side or an entire lower side. An outer wall of rotor iron core 211 is toothed with a uniform pitch and thus small teeth are provided to the outer wall. The position of the small teeth of the upper rotor iron-core is shifted from that of the lower iron-core in angles such that the peaks the teeth of the upper one correspond to the valleys of the teeth of the lower one.
A stator comprises stator core 203 and coils 204 wound on the salient poles of core 203. A rotor comprises magnet 201, upper and lower rotor iron-cores 211, and shaft 212 extending through magnet 201. Stator core 203 faces rotor iron-core 211 with a given space. Both the ends of shaft 212 are journalled by bearings 213.
In FIG. 32B, plural salient poles 205 provided on inner wall of stator core 203 are also toothed at the same pitch as rotor iron-core 211. These salient poles 205 are shifted given angles with respect to the small teeth provided to the outer wall of rotor iron-cores 211 of each phase. Powering the coils wound on the respective salient poles magnetizes small teeth on the powered salient poles. Thus the small teeth on the salient poles attract and repulse another small teeth, magnetized by magnet 201, on rotor iron-core, so that torque is generated for trying to fix the rotor at a given position. In this status, powering a phase of the coils is sequentially changed, thereby driving the rotor.
A smaller angular pitch of toothing rotor iron-core 211 and stator core 203 reduces a rotating angle of the rotor per phase change, so that a positioning at a finer angular pitch can be expected. As such, the stator and the rotor are toothed with fine pitches respectively, thereby increasing an angular resolution of an output shaft of the motor. As a result, a positioning at a fine pitch can be realized.
An inspiration from a stepping motor of the same type as discussed above has replaced an assembly of a toothed rotor iron-core and a magnet magnetized two poles along a shaft with a cylindrical magnet magnetized multi-poles. This idea has been developed to stepping motors with permanent magnets, and those motors are disclosed in Japanese Patent Gazette Nos. 3140814, 3071064, and 2740893. This idea has been also developed to a magnet rotary machine in Japanese Patent Application H10-80124 and to a two-phase motor with permanent magnets and disclosed in Japanese Patent Gazette No. 2733824. A three-phase stepping motor is one of the products developed from this idea and disclosed in Japanese Electric Academy Research Papers, section D, volume 115, second issue, published in 1995.
FIG. 33A and FIG. 33B show the rotor structure of a motor (a third example of conventional motors) similar to the three-phase stepping motor discussed above. FIG. 34 shows the rotor structure (a fourth example of conventional motors) described in the foregoing research papers.
This paper tells that the rotor structure shown in FIG. 33A and FIG. 33B produces the following advantages: (1) Magnetic flux distribution approximates to a sine wave. (2) An axial length of a magnetic pole can be extended as long as twice. (3) A magnetic path becomes two dimensional one, so that smaller magnetic resistance is produced. Those advantages make the motor shown in FIG. 33A and FIG. 33B produce an equivalent output to that of the motor shown in FIG. 34, and yet produces more accurate intermediate positioning in angles, i.e., where micro-stepping is activated, than the motor shown in FIG. 34.
On the other hand, there have been only few cases where the vernier structure is applied to brush-less motors. However, various techniques have been developed to enhance the performance of the brush-less motor, e.g., improving the performance of magnets, developing a low-loss material of the iron-core, improving a space factor of the windings for reducing copper loss. Recently a high-performance rare-earth oriented magnet formed of Neodymium-Iron-Boron (Nd—Fe—B) or Sm—Fe—N has been developed. The use of this magnet makes the iron-core become magnetically saturated because a saturation magnetic flux density of the iron core is low with respect to such a high performance magnet having more an improved magnetic flux density. As a result, the motor cannot use fully enough the improved performance of the magnet, so that the motor per se improves its performance only to some extent.
The prior art previously discussed has been developed for stepping motors in mind, and the vernier structure contributes mainly to an improvement of an angular resolution. This prior art is thus not optimized for an output density representing an output per unit volume, or volumetric efficiency representing how much a motor can be downsized maintaining its characteristics, so that this prior art, as it is, cannot apply to brush-less motor.
Further, a stepping motor needs always a constant current because of its current-carrying method, while a brush-less motor does not carry a current in a phase coil which produces no torque. Therefore, in general, the current-carrying method of the stepping motor has lower efficiency than that of the brush-less motor in operation.
What is more, the stepping motor is good at positioning control but is poor at torque control. A brush-less motor capable of torque control is thus employed for torque control in general.